FIG. 10 shows a prior art DA converter consisting of a PAM circuit 1 and a low pass filter 2, in which pulses for different bits of a digital signal D, which is to be converted into an analogue signal, are inputted to the PAM circuit 1; a pulse amplitude modulated signal P as indicated by (a) in FIG. 11 is outputted by the circuit; and low frequency components of the pulse amplitude modulated signal P are extracted through the low pass filter 2 having frequency characteristics indicated by (d) in FIG. 11 to obtain an analogue signal indicated by (b) in FIG. 11. Denoting the cut-off frequency of the low pass filter 2 by f.sub.c and the sampling frequency of the PAM circuit 1 by f.sub.s , f.sub.c =f.sub.s /2 is valid (by the sampling theorem) and the interpolation function by this low pass filter 2 can be represented by (c) in FIG. 11.
Consequently, when time sequential discrete components, t.sub.1, t.sub.2, t.sub.3 . . . etc. of the pulse amplitude modulated signal P indicated by (a) in FIG. 11 pass through the low pass filter 2 having the cut-off frequency f.sub.c =f.sub.s /2, as clearly seen from (c) in FIG. 11, the output for the pulse at t=t.sub.1 is zero at t=n.tau. (.tau.=1/f.sub.s) and when the pulse train of the pulse amplitude modulated signal indicated by (a) in FIG. 11 passes through the low pass filter 2 having the characteristics indicated by (d) in FIG. 11, the original signal represented by the pulse train described above is reproduced and it is smoothly completed also for the parts other than t=t.sub.1 .+-.n.tau., as indicated by (b) in FIG. 11.
However by the prior art DA converting method described above, as clearly seen from (b) in FIG. 11, ringing determined by the cut-off frequency f.sub.c of the low pass filter 2 is produced between t.sub.-2 and t.sub.-1 and between t.sub.-1 and t.sub.0.
In general, the part of the ringing generated by the DA converter is set at a part having no sensitivity for the sense of sight and the sense of hearing, when the signal is displayed on a play back device, but in the case where the cut-off frequency f.sub.c (f.sub.c =f.sub.s /2) is set very closely to the limit, it can produce often disturbance in the sense of sight and the sense of hearing.
Therefore the oversampling method has been proposed, by which the sampling frequency is equivalently increased to mf.sub.s (m.+-.2) by complementing vacant parts in the PAM signal and the cut-off frequency of the low pass filter is set below mf.sub.s /2.
By this method, even if ringing as described previously is produced, it is at a high frequency so that it produces no disturbance, when a signal obtained by restoring it is displayed.
However, together with non-linearity of the play back device, the high frequency ringing may worsen reproduction of low level signals (e.g. black level and silent parts).
(a) in FIG. 12 shows an intensity distribution of a stepwise original signal (step from black to white) and (b) in the same figure indicates an image, when it is reproduced on a display system. On the other hand, (c) in FIG. 12 shows a signal waveform, in the case where a converter having characteristics indicated by (b) in FIG. 11 is used for the DA converter and (d) indicates an image reproduced from the signal indicated by (c), from which it can be clearly seen that black and white stripes are produced in the dark portion.
Apart therefrom, there is known another prior art DA converter disclosed in JP-A-Hei 2-220537, which is a device comprising a digital signal processing circuit, which digital-processes an input signal to output two series of digital signals, whose phases are orthogonal to each other; two digital-analogue converters, which convert these two series of digital signals into respective analogue signals; two low pass filters, which remove respective harmonic components contained in outputs of these two digital-analogue converters, each of these two low pass filters including an MOSFET analogue filter having variable frequency characteristics; two multipliers, which multiply outputs of these two low pass filters by respective carriers, whose phases are orthogonal to each other; an adder, which adds outputs of these two multipliers; and a control circuit, which controls automatically the frequency characteristics of these MOSFET analogue filters by using clock signals, which are signals inputted to the digital signal processing circuit. However this converter has a drawback that the construction is very complicated and thus it is expensive.